This invention relates to antibacterial compounds. In particular, it relates to semi-synthetic .beta.-lactam antibiotics which possess a 4,7-bicyclic ring system and to a process for the preparation thereof. The known .beta.-lactam antibiotics, the penicillins and the cephalosporins, are fused bicyclic compounds possessing respectively the 4-membered .beta.-lactam ring fused to the 5-membered thiazolidine ring and a 6-membered dihydrothiazine ring. Monocyclic .beta.-lactams such as monobactam are known as are other bicyclic .beta.-lactams such as thienomycin and the clavulanic acids.
The compounds provided by this invention structurally comprise a 4,7-bicyclic ring system having the 4-membered .beta.-lactam ring fused to a 7-membered thiadiaza ring, formally designated as the 9-oxo-6-thia-1,3-diazabicyclo[5.2.0]non-3-ene bicyclic.